1. Field of the Invention
This invention relates to a fire-retardant electric device, particularly a transformer conforming with the standards of a fire retardant oil-immersed transformer as prescribed in Item 450-23 of U.S. NEC (National Electrical Code).
2. Prior Art
Fire-retardant transformers which have heretofore been put to practical use include a dry-type transformer, an insulating oil-free transformer such as a SF.sub.6 gas-filled one, and a transformer of the type in which the insulating material is impregnated with a fire-retardant insulating oil such as a silicone oil or a mineral oil having a high fire point.
They raise, however, problems as to their expensiveness or limitation on performance. In order to solve these problems, there was proposed the use of a tetraester, synthesized from pentaerythritol and a saturated fatty acid, as the electrical insulating oil (see Japanese Pat. Appln. Laid-Open No. 49-29500). The use of such an insulating oil alone, however, does not satisfy the requirements as prescribed in JIS (C-2320) particularly in respect of antioxidizing performance.